It has been known that a fatigue crack occurs in a welded portion of a structure such as, for example, a bridge, in association with use of the structure for a long period of time.
Here, various detection methods of detecting a deterioration of a structure have been proposed so far. However, various restrictions are imposed on these detection methods with respect to the installation height, state of the structure, and there may be a case where a crack occurring in the structure cannot be simply detected.